Iodine nutrition status and thyroid autoimmunity during pregnancy: a cross-sectional study of 4635 pregnant women

Background Pregnant women in Shanghai have long been at risk for mild iodine deficiency. Because thyroid autoimmunity in pregnant women can lead to premature birth and miscarriage as well as neurodevelopmental deficits in the fetus, the aim of this study was to explore the association of iodine nutrition status with thyroid antibodies during pregnancy. Methods A pregnancy-birth cohort was conducted including 4635 pregnant women in Shanghai, China. The eligible participants underwent a face-to-face interview and completed questionnaire surveys to collect baseline information and diet intake. Spot urine samples were collected to test urine iodine. Thyroid antibodies including thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TgAb) and thyrotrophic antibodies (TRAb) were tested. Single-factor analysis and logistic regression were used to evaluate the association between iodine status and thyroid autoimmunity during pregnancy. Results The median urinary iodine excretion level in the sample was 138.14 mu g/L (interquartile range [IQR] 80.90-219.00 mu g/L). Among all the subjects, 25.9% consumed non-iodized salt, 54.5% had iodine deficiency, and 31.0% had thyroid autoimmunity. The proportion of patients with iodine deficiency was significantly higher among those who consumed non-iodized salt (36.9% vs. 33.1%; p = 0.04). After adjusting for age, educational status, former smoker status, former drinker status, first pregnancy, and previous thyroid disease, non-iodized salt (odds ratio [OR] = 1.394 [confidence interval, CI, 1.165-1.562]; p = 0.003), iodine-rich food (OR = 0.681 [CI 0.585-0.793]; p = 0.003), iodized nutritional supplements (OR = 0.427 [CI 0.347-0.526]; p = 0.003), were found to be individually associated with thyroid autoimmunity in all participants. The results of the multivariable restricted cubic spline regression analysis showed a non-linear relationship between the continuous change in iodine intake and thyroid autoimmunity (p = 0.019). Participants with iodine deficiency (urinary iodine concentration, UIC,< 100 mu g/L) had an increased risk of testing positive for thyroid antibodies (TPOAb/TgAb/TRAb[+]; OR = 1.324 [CI 1.125-1.559]; p < 0.001). Moreover, this associated existed even after removing participants with previous thyroid disease. Conclusion Inadequate iodine nutrition in pregnant women is an independent risk factor for thyroid autoimmunity in Shanghai. It's important to maintain the adequate iodine status in pregnant women.